A field study of nearshore environmental changes in response to newly-built coastal structures in Lake Michigan

摘要

In this study, we monitored changes of cohesive nearshore environment including bluff and lake bottom/bed response to newly-built coastal structures with a thousand-meter-long revetment in Lake Michigan shoreline over a six-year study period. Sequential aerial photos showed that excessive slumping occurred only on the south bluffs and no bluff recession in the middle areas with coastal structures. Field measurements using our recently developed integrated geophysical techniques provided information on bathymetry, sand layer thickness, and lakebed downcutting over the nearshore reach of Concordia University in Lake Michigan. During the study period, the bathymetry profiles at the study site fluctuated dynamically, especially in the regions outside the shoreline structures, suggesting continuous and ongoing sediment erosion and deposition. The lakebed downcutting in front of the newly-built coastal structures is correlated with CWIH (cumulative wave impact height). Significant differences of lakebed downcutting in the north and south natural beach regions were revealed and may be associated with the nearshore sediment budget. The southwardly dominant longshore current maintains the equilibrium state of beach profiles in the north region, but the coastal structures prevent sediment supply from the well-protected bluffs in the middle region to the south region. The possible source of sediment supply in the south region is therefore from lakebed or bluff materials, supported by excessive bluff failures and lakebed downcutting. Overall the newly-built coastal structures seem to pose negative impacts on bluff stability at the south shore of the coastal structures.